- A new lab-based study has found that corals can convert oxybenzone, a common ingredient of sunscreens, into toxic chemicals.
- The toxins may be particularly harmful to bleached coral, which has expelled its symbiotic algae as a result of stressors such as rising sea temperatures.
- The results are controversial, however, because they may not reflect conditions in the real world.
- Oxybenzone in seawater at reefs may not reach high enough concentrations to cause any harm.
Coral reefs around the world face an uncertain future as a result of climate change,
Many tourists who swim at reefs use sunblock to protect their skin from the ultraviolet (UV) rays of sunlight, which can cause skin cancer.
A new study suggests that when corals are exposed to oxybenzone, which is an active ingredient of many sunscreens, this could make them even more vulnerable to rising sea temperatures.
The study has been published in Science.
However, coral experts have questioned whether the new findings, which involved exposing coral and anemones to oxybenzone in the lab, accurately reflect conditions in the real world.
In recent decades, rising sea temperatures have increased the frequency of mass bleaching events, which occur when coral polyps expel the algae that they rely on for survival.
Corals and algae collaborate in what biologists call a “symbiotic” relationship.
“These microscopic symbiotic algae give corals their coloration and, most importantly, much-needed nutrients,” said Dr. Victor Huertas, research associate at the ARC Centre of Excellence for Coral Reef Studies, part of James Cook University in Townsville, Australia.
“When the algal symbionts are expelled by the coral, the colony appears bright white because the coral tissue, which is transparent, now exposes the calcified skeleton that supports it,” said Dr. Huertas, who was not involved in the new study.
Bleaching is usually a response to the stress of raised temperatures, though there may be other causes.
“While bleached, corals may survive for up to a few weeks, although some corals tolerate heat stress better than others,” Dr. Huertas told Medical News Today.
The sale of oxybenzone-containing sunscreens has been banned in popular ecotourist destinations such as Hawaii and Key West, FL, in the United States.
The bans followed earlier research that implicated the chemical in coral bleaching, though scientists were unsure about the possible mechanism.
In the new study, scientists at Stanford University in Stanford, CA, found that oxybenzone is converted from a UV blocker to a “phototoxin” inside the cells of anemone and coral.
A photoxin is a chemical that becomes toxic when exposed to sunlight.
Their experiments also suggest that algae that live in the coral provide some protection against the toxins. This may mean that bleached coral is more vulnerable to the chemicals.
The researchers argue that, in a warming climate, oxybenzone-containing sunscreens could accelerate damage to coral reefs and hinder their recovery.
“[T]he finding that oxybenzone sunscreen is more toxic to these bleached anemones could suggest that it’s also more toxic to bleached corals, and that it would actually exacerbate these negative effects of warming … in the areas where you have human activity around,” first author and Ph.D. candidate Djordje Vuckovic told the Science podcast.
Vuckovic and his colleagues hope that their research will help guide the development of UV blockers that are less likely to be converted to phototoxins.
The researchers studied the sea anemone Aiptasia and the mushroom coral Discosoma.
In the lab, they exposed sea anemones to oxybenzone at a concentration of 2 milligrams per liter of seawater at 27°C.
Under artificial sunlight, which included UV light, all the anemones died within 17 days.
By contrast, when the scientists exposed the animals to simulated sunlight without oxybenzone, or to oxybenzone in the absence of any UV light, the mortality rate was negligible after 21 days.
The researchers report that while oxybenzone guarded tissues against the harmful effects of UV, both anemones and coral converted it into other molecules that were potent phototoxins.
But the symbiotic algae living inside the anemones appeared to protect them by storing or “sequestering” these phototoxins.
Anemones that lacked algae had higher mortality rates in the presence of oxybenzone, and their tissues contained higher concentrations of phototoxins.
The scientists found that algae inside Discosoma corals were even more effective at sequestering the phototoxins.
In their paper, they conclude:
“If the symbiotic algae of corals … protect them from the toxic effects of oxybenzone metabolites, then the widespread bleaching of corals in response to rising seawater temperatures will make them more susceptible … This may be a practical problem at reefs where recreational swimming and sunscreen use are high.”
However, coral experts expressed skepticism about these conclusions.
“The lab exposure, for 17 days, does not mimic how sunscreen leaves the skin of a reef tourist and is rapidly diluted,” said Dr. Terry Hughes, professor emeritus and former director of the ARC Centre of Excellence for Coral Reef Studies.
“The concentrations in the lab are far higher than previous measurements in the field,” he told MNT.
Prof. Hughes said previous bleaching events on the Great Barrier Reef in Australia closely match where the water was hottest for the longest.
“Even the most remote reefs in the world are now bleaching, far from any airport,” he added.
“If tourists want to save coral reefs, they should consider their carbon footprint and who they vote for, instead of avoiding sunscreen,” he said.
Prof. Hughes, who has written about the oxybenzone controversy for The Conversation, added that cancer specialists have warned about the dangers for public health if people stop using sunscreen.
“[W]hile I do not dispute the authors’ findings, I note that their study is based on experimental research conducted in a lab,” said Dr. Huertas.
“Things may change substantially out in the ocean, particularly when we have ocean currents at play,” he told MNT.
A study in Bermuda found this to be the case, he added.
“No sunscreens have been methodically studied with any standardization to determine if they are safe for the coral reef,” said Dr. Beth Goldstein, co-founder of Modern Ritual and president of Central Dermatology in Chapel Hill, NC.
She recommended mineral-based sunblocks as an alternative to those that contain oxybenzone or a similar chemical called octinoxate. But she emphasized that scientists have not proved other ingredients, such as preservatives and emulsifiers, to be safe for coral reefs.
On a recent family holiday her family opted for UV swimsuits, for instance, she told MNT.
“These are proven to be effective for long bouts in the sun without fear of ‘missing a spot’ when diving for sometimes hours at a time,” she said.